1. Field of the Invention
The present invention relates to wastewater treatment systems and methods, and, more particularly, to such systems and methods for wastewater treatment that are nonchemically based.
2. Description of Related Art
Wastewater treatment via xe2x80x9cnaturalxe2x80x9d means, i.e., without the addition of chemicals, has been accomplished with the use of aquatic and emergent macrophytes (plants) that, in concert with the attendant microorganisms and macroorganisms associated with macrophyte roots and stems, substantially mineralize biodegrade organic materials and substantially remove certain excess nutrients, such as nitrogen and, to a lesser extent, phosphorus. These macrophytes have typically been located in artificial marshlands, also known as constructed wetlands, which are designed for gravity flow. A negative aspect of such systems is that they are very land-intensive, requiring roughly on the order of 100 times as much land area as a conventional treatment plant, or, in terms of capacity, as much as 30-40 acres per 106 gallons of wastewater treated per day unless other treatment processes are incorporated into the constructed wetlands
Subsurface-flow wetlands, which comprise aquatic plants positioned above a gravel filter are also known for use in wastewater treatment. These systems have been shown to frequently fail, however. Failure is manifested as the upstream gravel tends to become clogged with biosolids, permitting the influent to bypass the clogged region and pass substantially untreated to a downstream region. Additionally, surfaced wastewater is a breeding ground for disease vectors and nuisance insects. Ultimately the gravel becomes so clogged that design wastewater treatment is substantially compromised. Plants also appear to have little treatment role in subsurface flow wetlands because the plant root systems are inhibited by conditions in the gravel filter from growing sufficiently long to extend into the gravel, and thus have minimal contact with the influent.
Several varieties of aquatic and emergent macrophytes are known to be used in wetland and aquatic wastewater treatment systems, including, but not limited to, cattails, bulrushes, sedges, and water hyacinths. In wetland treatment systems these plants may be packed in unlined or lined trenches or basins filled with a granular porous medium such as gravel or crushed stone. It has also been suggested to use recycled, shredded scrap tires in the place of the gravel. Another suggested wetland system variant is to place a semipermeable barrier between a lower level into which effluent enters and the plant root system for directing the waste waterflow across the entire plant bed.
In yet another variant floating aquatic macrophytes, typically water hyacinths, are placed in shallow lagoons where plant roots, with attendant microorganisms and macroorganisms, extending into the water column are a principal design treatment mechanism. Although this root zone treatment method can provide advanced secondary treatment effluent, its application is limited by climate to approximately 5% of the United States. The large treatment footprint of water hyacinth treatment systems prohibits enclosure in greenhouses for almost all economically viable applications.
It is also known to combine plant root zone treatment with conventional activated sludge technology. The principal advantages of combining root zone treatment with activated sludge are improved nutrient removal capability over root zone treatment alone and improved treatment stability in small, activated sludge treatment systems. Among the problems encountered with root zone/activated sludge technology is that the clarifiers employed do not scale well when the size of the system is reduced beyond a certain point. In addition, operator qualifications are high for activated sludge systems, adding to the expense of running the system. Root zone/activated sludge technology has been known to digest in situ a large fraction of the biosolids produced and maintained within the treatment system, thereby reducing system biosolids yield. The mechanism for yield reduction is thought to be the retention of biosolids flocs on plant roots with subsequent consumption and mineralization of flocs by the invertebrate community attendant to the root zone. Reduction of yield is desirable only to a certain point, however. As reactors in series are added, thereby increasing biosolids contact with the root zone, yield may be reduced to the point where an insufficient quantity of biosolids remain to be recycled from the clarifier to the reactors in series. Lack of recycled biosolids substantially degrades the treatment performance of the activated sludge treatment element. This design trap is inherent to root zone/activated sludge treatment systems. Preliminary studies have been performed on various aspects of the present invention by the inventors and other colleagues, as reported in xe2x80x9cFinal Report on the South Burlington, Vermont Advanced Ecologically Engineered System (AEES) for Wastewater Treatment,xe2x80x9d D. Austin et al., 2000; and xe2x80x9cParallel Performance Comparison between Aquatic Root Zone and Textile Medium Integrated Fixed Film Activated Sludge (IFFAS) Wastewater Treatment Systems,xe2x80x9d D. Austin, Water Environment Federation, 2001; both of these documents are incorporated herein by reference in their entirety.
The present invention provides a wastewater treatment system and method that are less land intensive than previous systems, as well as combining the advantages of a plurality of remediation techniques. The present invention has a smaller footprint than previously disclosed wetlands, reduces undesirable characteristics of an influent, and has a low yield, i.e., low proportion of matter needing disposal.
An additional feature of the invention provides a unified environment that includes a remediation system.
The wastewater treatment systems and methods of the present invention are amenable to the treatment of, for example, but not intended to be limited to, domestic wastewater, industrial waste or process water, stormwater, urban runoff, agricultural wastewater or runoff, and even biological sludges. The systems are capable of handling a flow range of approximately 2000-2,000,000 gal/day. The types of contaminants that can be treated in the system include suspended particles, nutrients, metals, simple organics (oxygen-demanding substances), and synthetic or complex organics. The undesirable characteristics typically desired to be remediated include, but are not intended to be limited to, average biological oxygen demand (BOD), average total suspended solids (TSS), total nitrogen, and concentration of oil and grease. The systems of the present invention can reduce BOD to  less than 10 mg/L, TSS to  less than 10 mg/L, and total nitrogen to  less than 10 mg/L.
The water treatment system of the present invention comprises a wastewater inlet, a treated water outlet, and a plurality of treatment modules between the inlet and the outlet. Each module is for treating the water with a selected process. Each module is in fluid communication with at least one other module for permitting sequential treatment of the wastewater by a plurality of processes.
Influent wastewater is first directed to a a pretreatment process, such as covered anaerobic reactor or screening process, which serves to perform an initial organic and solids removal. A means is provided for removing odors from gases or fumes that are produced herein.
Following pretreatment for a predetermined period, the wastewater is channeled to a fixed-film reactor, which achieves removal of organics and solids and denitrification. This fixed-film reactor is characterized in a low yield unit process, in which yield is, defined as kilograms of VSS exiting the system divided by kilograms of BOD entering the system. In alternate embodiments of the system, plural fixed-film reactors may be provided in series.
A given fixed-film reactor may operate in a substantially aerobic or anoxic mode, or alternate between anoxic and aerobic modes. A given fixed film reactor may operate as a complete or partial mix bulk liquid reactor. Alternatively, the reactor may operate in a fill and drain mode.
Water exiting the fixed-film reactor then is pumped or flows by gravity to a hydroponic reactor, in which aquatic plants are suspended atop the liquid for achieving aquatic-root-zone treatment. The hydroponic reactor is substantially aerobic, and operates to achieve VSS digestion in addition to continued removal of nutrients started in the fixed-film reactor. The hydroponic reactor has an inlet and an outlet. Herein the term hydroponic reactor will be taken to comprise aerated reactors that have a rigid rack set at the water surface to support plants that send down roots into the wastewater column. The rack covers substantially the entire water surface. Plants substantially cover the surface of the rack. In alternate embodiments plural hydroponic reactors in series may be provided.
Digestion of VSS in the hydroponic reactors, or plurality of reactors, is designed to produce an hydroponic effluent VSS concentration of approximately 50 mg/L or less, which is sufficiently low to permit economically viable filtration. Filtration of hydroponic effluent to advanced standards may be achieved by a plurality of technologies.
Recycling may be directed to an anaerobic pretreatment module or to the fixed-film reactor, prior to channeling water exiting the hydroponic reactor to a filtering and disinfection module.
The features that characterize the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description used in conjunction with the accompanying drawing. It is to be expressly understood that the drawing is for the purpose of illustration and description and is not intended as a definition of the limits of the invention. These and other objects attained, and advantages offered, by the present invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawing.